--- a/xpcom/tests/gtest/TestTimers.cpp
+++ b/xpcom/tests/gtest/TestTimers.cpp
@@ -61,17 +61,17 @@ public:
     mReentrantMonitor = new ReentrantMonitor("TestTimers::AutoMon");
     MOZ_RELEASE_ASSERT(mReentrantMonitor, "Out of memory!");
   }
 
   ~AutoCreateAndDestroyReentrantMonitor() {
     delete mReentrantMonitor;
   }
 
-  operator ReentrantMonitor* () {
+  operator ReentrantMonitor* () const {
     return mReentrantMonitor;
   }
 
 private:
   ReentrantMonitor* mReentrantMonitor;
 };
 
 class TimerCallback final : public nsITimerCallback